Scientists have unearthed a new species of single-celled animals
called Pfiesteria piscicida. These perpetrators are the culprits
for the many deaths among a billion fish in the eastern U.S. waters.
The fish are left with bloody sores after the large groups of animals attack
them. One of the discoverers was JoAnn Burkholder of North Carolina
State University. She helped them name the species, which means "Fish
Killer".
Buckholder thinks that Pfiesteria originally fed on bacteria
and algae, but then later turned to larger prey for a good reason.
The recent farm and development around the area caused an abundance of
nutrients to be poured into the rivers. This made a shift in their
feeding habits enabling them o attack in a group for fish. Humans
who have been exposed to this organism have developed the same type sores,
severe memory loss, and other cognitive problems.
It has been found that Pfiesteria is so dominant because of its complex
life cycles. It has 24 known forms. It begins as a dormant
cyst living on the bottom of the river until the waste of a dead fish triggers
if to change to a toxic form. Then they propel themselves with their
whip-like tails to poison the fish with their venom. As the fish
die, Pfiesteria reproduce and feed on the fish's fluids. Fresh cysts
then fall to the bottom to await new victims. The amoeba-like forms
have a shell that armors it against microbial predators. When in
this form, they scavenge on dead fish later changing to prey on bacteria
and algae. At other times, Pfiesteria extracts the chloroplasts from
algae and uses the photosynthesis to supplement their food supply.
Thus, this masquerades this animal as a plant.

From Discover, The Jaws You Can't See by Catherine Dodd, January 1998,
p. 102-103.